(a) Field of the invention
The present invention relates to a method for fabricating thin film transistor, more particularly, to a method for fabricating thin film transistor which not only manufactures a polycrystalline silicon layer having large grain size and containing a trace of residual metal catalyst by heat treating thereby crystallizing the metal catalyst layer after forming an amorphous silicon layer on a substrate, forming a capping layer formed of nitride film having 1.78 to 1.90 of the refraction index when crystallizing the amorphous silicon layer and forming a metal catalyst layer on the capping layer, but also controls characteristics of the polycrystalline silicon layer by controlling the refraction index of the capping layer.
(b) Description of Related Art
In a thin film transistor used in display devices, a semiconductor layer is generally formed by crystallizing the dehydrogenated amorphous silicon layer after depositing amorphous silicon on a transparent substrate such as glass or quartz and dehydrogenating the amorphous silicon layer.
A semiconductor layer composing source, drain and channel region of the thin film transistor is formed by depositing an amorphous silicon layer on a transparent substrate such as glass by using chemical vapor deposition method. However, silicon directly deposited on a substrate by methods such as chemical vapor deposition method has low electron mobility by forming an amorphous silicon layer containing about 12% of hydrogen content, and the silicon layer is damaged by burst of hydrogen contained in the silicon layer when crystallizing the amorphous silicon layer into a crystalline silicon layer having high electron mobility by heat treating the amorphous silicon layer having low electron mobility. The dehydrogenation process is proceeded to prevent burst phenomena of hydrogen generated during crystallization, wherein the dehydrogenation process is performed by heat treating the amorphous silicon layer generally in a furnace at temperature of about 400° C. or more for several minutes to several hours. After performing the dehydrogenation process, the crystallization process for crystallizing the dehydrogenated amorphous silicon layer is executed.
Methods for crystallizing the amorphous silicon layer into a polycrystalline silicon layer include solid phase crystallization method, excimer laser crystallization method, metal induced crystallization method and metal induced lateral crystallization method, wherein the solid phase crystallization method is a method for annealing an amorphous silicon layer for several hours to tens of hours at temperature of about 700° C. or less, that is, a deformation temperature of glass for forming a substrate of display device in which thin film transistor is used, the excimer laser crystallization method is a method for crystallizing the silicon layer by injecting an excimer laser into silicon layer, thereby partially heating the silicon layer to a high temperature for a very short time, the metal induced crystallization method is a method for using a phenomenon in which phase transition of amorphous silicon layer into polycrystalline silicon layer is induced by the metal by contacting metals such as nickel, palladium, gold and aluminum with amorphous silicon layer or injecting the metals into the amorphous silicon layer, and the metal induced lateral crystallization method is a method for crystallizing silicon layer by using a method for sequentially inducing crystallization of silicon as silicide produced by reacting metals and silicon is being laterally spread continuously.
However, the solid phase crystallization method has demerits that deformation of substrate is easily generated since process time is too long, and heat treatment is performed at high temperature for long time, the excimer laser crystallization method has demerits that an expensive laser equipment is required, and protrusions are formed on the surface of polycrystalline silicon layer so that characteristics of interface between semiconductor layer and gate insulation film are deteriorated, and the metal induced crystallization method and the metal induced lateral crystallization method have demerits that a large amount of metal catalyst is remained on polycrystalline silicon layer crystallized so that leakage current of semiconductor of thin film transistor is increased.